Conventionally, a Mach-Zehnder interferometer has been used for an optical modulator that modulates light emitted from a light source in some cases. In such an optical modulator, signal electrodes and grounding electrodes are arranged along optical waveguides parallel to each other. In recent years, an optical modulation system has been diversified, and the optical modulator has been provided with a plurality of Mach-Zehnder interferometers in many cases. In this case, the Mach-Zehnder interferometers are integrated into one chip, thus enabling the size of the optical modulator to be reduced.
The optical modulator provided with the Mach-Zehnder interferometers inputs therein a plurality of electrical signals that are different from each other, thus enabling generation of a multi-level modulation signal. That is, the electrical signals that are different from each other are input to the respective signal electrodes corresponding to the respective Mach-Zehnder interferometers from the outside of the optical modulator, thus enabling optical modulation by a multi-level modulation system, such as a differential quadrature phase shift keying (DQPSK) system, for example.
There may be a case in which connectors are provided to an input part from which electrical signals are input to the optical modulator. However, if connectors are provided for respective electrical signals, the optical modulator becomes larger in size thus increasing a mounting area for the optical modulator. To address this issue, there may be a case in which a flexible printed circuit board (FPC) having flexibility is used for the input part from which electrical signals are input to reduce the size of a device including the optical modulator.
To be more specific, a plurality of circuit patterns corresponding to the respective signal electrodes of the optical modulator are printed on the FPC, and an electrical signal output from a driver is input to the optical modulator via each circuit pattern printed on the FPC. One end on the optical-modulator side of the FPC is inserted into a cutout portion formed in the optical modulator, and each circuit pattern is, for example, soldered onto a coaxial terminal projecting to the inside of the cutout portion so as to be electrically connected with the optical modulator. On the other hand, one end on the driver side of the FPC, each circuit pattern of which is, for example, soldered onto an electrode pattern for transmitting an electrical signal from the driver, is electrically connected with the driver.
In terms of the reduction in the size of the device, such a structure may be adopted that arranges the optical modulator and the driver hierarchically by using boards that are different from each other to connect the optical modulator and the driver that are hierarchically arranged with each other, by using the FPC (see Japanese Laid-open Patent Publication No. 2005-128440).
However, in the structure in which the optical modulator and the driver that are hierarchically arranged are connected with each other by using the FPC, the arrangement space of the optical modulator and the arrangement space of the driver are separated from each other and hence, there exists the possibility that the entire mounting area of the device increases. Consequently, the structure in which the optical modulator and the driver are hierarchically arranged is unpractical.
It is also possible to adopt such a structure that a part of the driver is housed inside the cutout portion included in the optical modulator to reduce the mounting area corresponding to the driver. However, in this case, the coaxial terminal projecting to the inside of the cutout portion and the driver are arranged in close proximity to each other thus giving rise to a sharp flexure of the FPC that connects the coaxial terminal and the electrode pattern extending from the driver. When the FPC is sharply flexed, unintended stress is applied to the FPC and hence, there exists the possibility that disconnection occurs in the connection portion between the coaxial terminal and the FPC or in the connection portion between the electrode pattern and the FPC.